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[dm-devel] Re: [k-ueda@ct.jp.nec.com: Re: request-based dm-multipath]

Mikulas Patocka wrote:
>>> What is the exact reason for your patch? I suppose that it's some 
>>> performance degradation caused by the fact that dm-multipath doesn't 
>>> distributes requests optimally across both paths. dm-multipath has 
>>> pluggable path selectors, so you could improve dm-round-robin.c (or write 
>>> alternate path selector module) and you don't have to touch generic dm 
>>> code to solve this problem.
>>> The point is that improving dm-multipath target with better path selector 
>>> is much less intrusive than patching device mapper core. If you improve 
>>> dm-multipath target, only people hacking on dm-multipath will have to 
>>> learn about your code. If you modify generic dm.c file, anyone doing 
>>> anything on device mapper must learn about your code --- so human time 
>>> consumed in much worse in this case.
>>> So, try the alternate solution (write new path selector for dm-multipath) 
>>> and then you can compare them and see the result --- and then it can be 
>>> consisdered if the high human time consumed by patching dm.c is worth the 
>>> performance improvement.

The main purpose of request-based dm patches is to provide a framework
to implement optimal path selectors in non-intrusive way.

As you mentioned, it might be possible to implement a good path
selector at bio-level by checking internals of underlying devices' 
request queues and/or I/O schedulers.

However, requiring knowledge/assumptions of internals of other layer
is not a right approach.
Or, splitting the request-based multipath driver out of dm would trash
the existing userspace tools and libraries, so that's not good either.
Thus we chose the design of 'adding request-based mapping feature to dm'.
It doesn't break existing target drivers and userspace tools.
The feature is enabled only for request-based target drivers.
If you want to add a bio-specific new feature, it's still possible.

The design has been discussed in several mailing lists, Ottawa Linux Symposium
and Linux Storage/Filesystem workshop including maintainers
of related subsystems for these few years,
and I think we got basic agreement on this direction.

As for your barrier works, we are looking into the patches
to make them work on request-based dm as well.

Jun'ichi Nomura, NEC Corporation

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